Various monitoring technologies are known and used to monitor the location of an article or to provide identification in a wide range of contexts. One such technology, known as “tagging,” is commonly employed, for example, in shoplifting security systems, security-badge access systems and automatic sorting of clothes by commercial laundry services. These conventional tagging systems may use some form of radio-frequency identification (RF-ID). In such systems, RF-ID tags and a tag reader (or base station) are separated by a small distance to facilitate near-field electromagnetic coupling therebetween. Far-field radio tag devices (far-field meaning that the sensing distance is long compared to the wavelength and size of the antenna involved) are also known and used for tagging objects at larger distances.
The near-field coupling between the RF-ID tag and the tag reader may be used to supply power to the RF-ID tag (so that the RF-ID tag does not require a local power source) and to communicate information to the tag reader via changes in the value of the tag's impedance. In particular, the RF-ID tag incorporates an active switch, packaged as a small electronic chip, for encoding the information in the RF-ID tag and communicating this information via an impedance switching pattern. The impedance directly determines the reflected power signal received by the reader, and as a result, the RF-ID tag is not necessarily required to generate any transmitted signal.
It is desirable in commercial applications or technology to reduce the cost of RF-ID tags. Though simple in principle, RF-ID tags may require sophisticated manufacturing techniques to produce. A more economical alternative involves marker elements adapted to affect an interrogation signal in a measurable, characteristic way. Many such systems utilize magnetic or magnetomechanical tags. For example, a magnetic wire or strip exhibiting harmonic behavior may be stimulated within an interrogation zone by transmitter antenna coils. The coils generate an alternating magnetic interrogation field, which drives the marker into and out of saturation, thereby disturbing the interrogation field and producing alternating magnetic fields at frequencies that represent harmonics of the interrogation frequency. The harmonics are detected by receiver antenna coils, which may be housed in the same structure as the transmitter coils. Accordingly, the appearance of a tagged article within the zone—which may be defined, for example, near the doors of a retail store or library—is readily detected.
Inexpensive, magnetic antitheft systems tend to encode very little, if any, information. Essentially, the tag merely makes its presence known. Although some efforts toward enhancing the information-bearing capacity of magnetic tags have been made—see, e.g., U.S. Pat. Nos. 5,821,859; 4,484,184; and 5,729,201, which disclose tags capable of encoding multiple bits of data—the tags themselves tend to be complex and therefore expensive to produce, and may require special detection arrangements that limit the interrogation range (the '859 patent, for example, requires scanning a pickup over the tag) or involve specialized equipment.